This invention relates to methods for gettering heavy metals and, more particularly, to a new and improved method for gettering heavy metals from a single crystal silicon plate to eliminate contaminants introduced during the processing of the silicon plate and the resultant microscopic defects.
The trend toward reduction of size and increased integration in electronic devices demands large scale integration (LSI) techniques which require more complete purification of products. To improve the degree of purification of such products, production is carried out in an ideally clean environment and chemicals of high purity are used. Despite such precautions, however, contamination of the products to some extent is inevitable. It has been reported that such products suffer from various induced defects which are caused in the oxidation and diffusion stage of processing and from contamination with heavy metals in the stage of controlled orientation of crystal growth. Examples of the heavy metals responsible for the contamination include Fe, Cu, Ni, Cr, Co, and Au. The resulting defects cause various problems such as shortened life time of MOS's and decreased memory holding time of RAM's and, consequently, they strongly affect the performance and yields of the devices containing them.
This contamination with heavy metals may be diminished to a certain extent by efforts at purification but some contamination is inevitable. For the purpose of avoiding the effects of such contamination, various procedures have been tried which produce deformation of the rear surface of a silicon wafer, thereby causing the entrapped heavy metal to be attracted to the crystal defect caused by the deformation and enabling the activated region of the device to be kept in a defectless state. These are as follows:
(1) Scratches are inflicted on the rear surface of wafer by lapping or marking. PA0 (2) Strain is introduced by exposure to a laser beam. PA0 (3) Strain is introduced by injection of ions. PA0 (4) A Si.sub.3 N.sub.4 film is formed on the rear surface of the wafer.
The infliction of scratches on the rear surface of a wafer as by lapping in procedure (1) is disadvantageous because the treatment must be performed on each wafer separately, the lapping work consumes much time, and the treatment for cleaning the lapped wafer is complicated. The vacuum deposition by means of a laser beam in procedure (2) turns out to be an expensive operation because the entire surface of each wafer must be exposed to the laser beam. In the case of procedure (3), requiring injection of ions, control of exact doses of ions is difficult and the wafer requires complicated treatments such as heat treatment after the injection. The method of procedure (4), involving the formation of an Si.sub.3 N.sub.4 film, is optimum only when the thickness of the film is 2000 .ANG.. If the thickness exceeds 2500 .ANG., the film thickness must be controlled carefully because the wafer may warp or there may be slippage between the layers. Moreover, this method is not as effective as might be expected unless it is carried out in conjunction with a gettering treatment involving diffusion of phosphorus in the rear surface of the wafer. Since Si.sub.3 N.sub.4 forms an insulating film on the wafer, this method inevitably requires an extra step for the removal of Si.sub.3 N.sub.4.
An object of the present invention is to provide a convenient and highly effective method for gettering a heavy metal from a silicon plate which overcomes the disadvantages of the prior art.